The present invention relates to methods and apparatus for producing refrigeration and more particularly to methods and apparatus for refrigerating a chamber by discharging carbon dioxide therein.
Numerous methods and systems for providing refrigeration to a chamber have been developed. One common type of system is a "mechanical" refrigeration unit in which expansion of a circulating working fluid, such as a fluorinated halocarbon, effects a cooling of such fluid so that upon heat exchange between air and the fluid, air may be chilled and in turn be utilized to refrigerate a chamber. The motive power for such systems is typically supplied by an internal combustion engine or electric motor. Other systems include apparatus for discharging a cryogenic fluid such as liquid nitrogen (typically at -320.degree. F.) into a chamber or discharging liquid carbon dioxide from a nozzle or "snow horn" into a chamber. Such latter discharge or expansion results in the conversion of liquid carbon dioxide to solid and gaseous phases (-109.degree. F.) although fans are required to suspend particles of solid CO.sub.2 until the same sublimate in order to obtain uniform refrigeration of a particular chamber. Still another type of refrigeration system includes a coil or other vaporizer for vaporizing liquid nitrogen or liquid carbon dioxide to drive a pneumatic motor which in turn drives a fan to blow air across the coil containing such liquid thereby cooling the air which is then utilized to refrigerate a chamber. The nitrogen or carbon dioxide gas is simply vented after driving the pneumatic motor.
Although the aforedescribed systems will operate to produce refrigeration, each has one or more drawbacks that tend to restrict its use. For example, the "mechanical" refrigeration units require relatively high capital costs as a result of utilizing a compressor and expander, etc. Furthermore, when such units are subjected to substantial vibration as will occur during use with over the road trailers, equipment reliability is less than desirable and maintenance costs are relatively high. Mechanical refrigeration units are heavy and when driven by internal combustion engines as is typically the case when used with over the road trailers, such systems are noisy and release undesirable exhaust emissions. Liquid nitrogen refrigeration systems require a supply of this cryogen and are expensive to operate. In the course of filling storage vessels with liquid nitrogen, nitrogen gas must be vented thereby causing substantial losses of this cryogen and increased overall costs. In addition, due to the low temperature (-320.degree. F.) of liquid nitrogen, storage vessels require considerable insulation and must be constructed of relatively expensive materials, e.g. stainless steel. Use of liquid nitrogen systems does not result in circulation of the atmosphere of the chamber in which such systems are disposed and frequently results in intense localized cooling. Although snow horns or nozzles are effective to discharge a stream of solid and gaseous carbon dioxide, these devices are generally not effective to uniformly refrigerate a chamber due to the tendency of the solid carbon dioxide (snow) to settle on the chamber floor or contact items being refrigerated therein without the use of elaborate control systems requiring external power sources. Although fans may be disposed in the chamber to maintain the snow suspended in the atmosphere thereof until the snow sublimates, fans represent costly additional equipment. Furthermore, as fans perform work in the chamber, heat is effectively supplied to the chamber thereby resulting in a greater consumption of liquid carbon dioxide in order to maintain a predetermined temperature in the chamber.
Accordingly, a clear need exists for a refrigeration system, which is structurally simple, light in weight, clean with respect to the emission of pollutants, quiet and reliable in operation even when subject to substantial vibrations and which is yet effective to uniformly refrigerate a chamber without consuming excessive quantities of refrigerants.